1. Field of the Invention
This invention relates to novel configurations for the probe structure of an air-gauge nozzle used in the microlithographic fabrication process of miniaturized devices and, more particularly, such probe-structure configurations used to achieve precise pole trimming as a step in the microlithographic fabrication process of miniaturized components (e.g, thin-film read/write heads for disk drives).
2. Description of the Prior Art
The concept of using air gauges for measuring mechanical dimensions has a long history. In a pressure-type air gauge (see FIG. 5), air from a constant pressure supply P.sub.0 flows through an adjustable orifice, and then through a circular air-probe structure at the output tip of a nozzle which is placed in close proximity to a workpiece. The pressure measured by a gauge placed between the orifice and the output tip of the of nozzle's air-probe structure varies in inverse proportion to the distance between this output tip and the workpiece. With suitable plumbing and gauging, spacing between the air-probe structure's output tip and the workpiece can be measured with a precision better than 1 .mu.m.
In practice, the variable orifice is set to a value such that the measured pressure P.sub.out is approximately mid-range when the air probe's output tip is at its nominal working distance with respect to the workpiece. If the workpiece were to come in contact with the air-probe structure's output tip of the nozzle, the pressure would rise to the supply pressure P.sub.0. When the workpiece is removed, the gauge pressure drops to a low value, which is not zero because of the back-pressure inherent in the nozzle. In practice, the air gauge is typically operated with the air-probe structure's output tip and workpiece spacing in a near-linear region between these extremes.
The electrical analogue to the air gauge (see FIG. 6) is a serially connected voltage divider. Constant pressure is modeled by a constant electromotive force V.sub.0, the orifice by an adjustable resistor R.sub.1, and the nozzle by a variable resistor R.sub.2. The latter is a combination of the fixed resistance of the nozzle's air-probe structure itself with a resistance which varies with the nozzle-workpiece spacing. This variation can be determined empirically.
A nozzle's air-probe structure output tip in contact with the workpiece is modeled by an infinite impedance R.sub.2, and in that situation the output voltage V.sub.out would rise to the supply voltage. The air capacity of the system between the orifice and the air-probe structure's output tip is modeled by a capacitance C.sub.out in parallel with R.sub.2. The time response of the measurement system thus depends on the values of R.sub.1, R.sub.2, and C.sub.out. Therefore, it is desirable to minimize the values of R.sub.1, R.sub.2, and C.sub.out to achieve a fast response. In optical microlithography, as known in the art, an image of tiny spatial patterns is projected on a resist-coated surface of an object. The object may be located on a stepper having six degrees of freedom that employs servo-control means to precisely position the object so that the focal plane of the projected image of tiny spatial patterns coincides with the object's resist-coated surface (i.e. the object's resist-coated surface is positioned within the limited depth of focus of an optical lithography projection system).
The air gauging technique may be used to measure the height of a workpiece, such as a resist-coated wafer in an optical lithography tool. In typical applications, such as semiconductor microlithography, the area sensed by the air gauging technique is not generally an issue. This "footprint" area is roughly equivalent to the inner diameter of the nozzle which defines the area of air probe output tip, and the reading is proportional to the average height over that area.
Another application for optical microlithography is the fabrication of thin-film read/write heads for disk drives, which employs a "rowbar" technique for the step of trimming the poles of otherwise already-fabricated disk-drive thin-film heads. In this "rowbar" technique, the spatial resolution becomes a concern. Rather than averaging the height over an extended area, it is desirable to sense the height at a specific location over a small area.
For example, the original full-size Winchester thin-film head had dimensions of 0.220.times.0.135.times.0.076 inches (i.e., 5.59.times.3.43.times.1.93 millimeters). However, several generations of miniaturization of thin-film heads have taken place. In the so-called pico-sized thin-film heads, the dimensions of each head has been reduced to only 0.049.times.0.039.times.0.012 inches (i.e., 1.24.times.0.99.times.0.305 millimeters (mm)). One way to provide an air gauge nozzle. having an inner nozzle diameter sufficiently small for the step of trimming the poles of pico sized thin-film heads is to simply scale down the inner probe diameter to the desired size so that the area sensed by the air probe output tip of the nozzle is adequately small. The drawback of this approach is that the back pressure of the air probe can become extremely high, which compromises the ability to make accurate measurements. In addition, the flow rate is low, and the response time for a gauging system which monitors pressure can become prohibitively slow. Furthermore, the nozzle's output end can become so small that it is prone to mechanical damage which may be caused by inadvertent contact with the workpiece surface.